Method and apparatus for reducing the contact drop between current-carrying contents



Aug. 15, 1933. R. M. BAKER ET AL 1,922,191

METHOD AND APPARATUS FOR REDUCING THE CONTACT DROP BETWEEN CURRENTCARRYING CONTENTS Filed Dec. 1, 1951 Fig. 1.

e0 70 80 9a /00 /0 20 30 40 .50 m0 WITNESSES: p r s Amperes INVENTORS'.

ATTORNEY H I I 1,922,191

1,922,191 7 p 1 "rmrnoninretnmmms roe. aEnUorNG THECONTA'CT DROP BETWEENCUR- EEN 'F'E B EN a RobertlvisBaker'and' Lloyd smeaa wiikinsburg;

Pa;,;:assignors;-to WestinghouseElectric and i, Manufact rjrlg Company,a; Corporation of fAppli cation'Decembeir 1, 19 31.';-- Serial No.578,330

7bliinslllcLineszs) Our invention relates','to a method ,and .ap'paeratus for reducing the contact; drop bettyeen ctu'rent-carry'ingcontents, and Iparticul'arly bee tween the brush andcurrent-'carrying'rings or commutator "of 'a' dynamo-electric machine; IInith'e operation of current-carrying elements such as pomposition"brushesv known as carbon brushes, jandtheir cooperating'c'ommutators orslip,f1i11gs,' considerable loss .is introduced by a voltagedropbetwe'en the brushfa'ndithe cooper ating element. Forfmanyuses suchas relays, thisvoltage drop is a serious'detiiment to the sensitivity ofthe device. In other. devices, such as 1 dynamo-electric machines, thecontact drop introduces "serious heating in thecurrent-carrying'devicesf" 'Itis an object 'of our inventionto provide acurrent-collecting device having a very low contact drop, between thecarbon current-carrying element and the cooperating metallic element. Wehave accomplished this result by providing an inert or non-oxidizinggaseous medium surrounding the current-collectingdevicesandintroducing asmall quantity of mercury vapor into the non-oxidizing gas. Experimentshave shown that metals or metal alloys capable of amalgamating withmercury produce the lowest contact drop. While non-amalgamating metalshave substantially the same contact drop as in air. However, such smallquantities of mercury are necessary, that, after several monthsoperation, no trace of amalgamation can be noted.

Other objects and advantages of our invention will be apparent from thefollowing detailed description, taken in conjunction with theaccompanying drawing, in which:

Figure 1 is a view, partly in section and partly in side elevation of adynamo-electric machine, constructed according to our invention, and;

Fig. 2 is a volt-ampere characteristic curve for a brass ring both inair and in the inert medium according to our invention, and

Fig. 3 is a similar volt-ampere curve for a copper slip-ring.

The apparatus according to Fig. 1 comprises a dynamo-electric machine 1,such as a rotary converter, having both slip-rings 4 and a commutatingcylinder 5, co-operating with suitable composition brushes 6 usuallyknown as carbon brushes, the entire machine being enclosed in a suitableair-tight housing 10 in which is placed a suitable non-oxidizing coolingfluid, such as hydrogen or nitrogen. At a convenient point in the casing10 we provide a receptacle 12 for containing liquid mercury 13.Preferably, this receptacle 12 should be placed on some portion of themachine, which, during normal operation, will have a temperature of 40to 59, as we have found that the vapor pressure of mercury at thistemperature will provide sufficient mercury vapor to produce the desiredlow contact drop.

In the operation according to our invention,

the active air aboutthe machine, and particularly about the commutatingelements, is replaced by a suitable inert gas. which is in contact withthe mercury in the receptacle. A small amount of the mercury willevaporate into the inert gas and probably will be deposited on thesurface of the metallic current-carrying element. In any event, we havefound that carbon brushes operating against suitable metallic elementshave a-contact drop much less than the same elements when operated inair. .,.,B.efei'ring to Fig. 2, it will be noted that for substantialcurrents, the contact drop was greater'than one volt when operated inair, but, when operated in hydrogen in the presence of mercury vapor,the maximum contact drop was less than .1 volt.

Also referring to Fig. 3, it is noted that while the contact drop for acopper ring in air was well above one volt throughout a wide currentrange, the maximum drop in hydrogen charged with mercury according toour invention was less than .5 of a volt. Attention is also directed tothe fact that the volt-ampere characteristic of the contact drop in aninert gas is substantially a straight line function, in other words,substantially a pure resistance function, while the contact drop in airrapidly reaches a maximum and then flattens out in a manner totallydifferent from a pure resistance function.

In normal high-current work, such as slip-rings and commutators, theelimination of the contact drop loss permits much higher currentdensities in the current-collecting device without overheating the samethan would otherwise be possible.

t has long been known that dynamo-electric machines can be operated at ahigher rating When cooled by hydrogen or other gas having a high thermalconductivity. However, in many machines, such as rotary converters, ithas been impossible to take advantage of hydrogen cooling, because suchmachines use so-called metallized brushes in order to maintain asuitable lowcontact drop between the brushes and slip-rings. Thesebrushes, however, wore away rapidly and deposited finely dividedparticles adjacent to the slip-rings. Since it was necessary tofrequently clean the device, it was inadvisable to put a casing aboutthe machine. However, with our invention, suitable low-contact drops maybe maintained using so-called carbon brushes which are not subject tothe dusting 'above referred to.

In certain applications where the dusting of metallized carbon brushesis not objectionable the contact drop of such brushes can be stillfurther reduced by the application of our invention.

We have also found that the current collecting devices of relays andsuch like equipment may be made very sensitive by the use of ourimproved method of low voltage current transfer.

We have found that as little as two grams of mercury per thousand cubicfeet of inert gas is quite adequate to secure thebenefitsof ourinvention While this amount of mercury is quite inadequate to injure thewinding of the machine or be injurious to workmen or others about themachine. V I While we haveshown and described a specific embodiment ofour invention, it is apparent that many changes and modifications can bemade therein without departing from the spirit and scope of'ourinvention. We desire, therefore, that only such limitations shall beimposed'as are embodied in the accompanying claims or as may benecessitated by the prior art.

We claim as our invention:

1. A method of producing a low contact drop between a brass contactmember and a carbon contact member which comprises maintaining an inertgas about the contact members and supplying a small quantity of mercuryvapor to the contact members.

2. A current-collecting device comprising a rotating current-carryingring, a carbon brush in contact with the ring, a casing enclosing thering and the brush, an inert gas in the casing, and a pool of mercuryenclosed in said' casing and in contact with the gas in said casing.

3. A current collecting device comprising a metal current carryingelement, the metal of said currentcarrying element-having an afiinityfor mercury, a second current-carrying member having sliding contactwith the first mentioned member, a casing enclosing both of saidmembers, a non-oxidizing gas in said casing about the current-carryingmerr'ibers and a small amount of mercury vapor in said casing.

4. A current collecting device comprising a .brass current carryingelement, a carton brush in contact therewith, a casing surrounding thedevice, an inert gas filling said casing and means for supplying mercuryvapor in said inert gas.

5. A current-collecting device comprising a current-carrying ring, saidring being of a material capable of amalgamation with mercury, acurrent-carrying brush in contact with the ring, a casing surroundingthe collecting device, a non-oxidizing gas in the casing and means forintroducing mercury vapor into the gas.

6. A current-collecting device comprising a copper containingcurrent-carrying element, a metaiized carbon brush cooperatingtherewith, a casing enclosing the current collecting device, hydrogen insaid casing and about the currentcollecting device and means forsupplying mercury vapor in the hydrogen.

-'7. A current-collecting device comprising a current-carrying elementof a material capable of amalgarnating with mercury, ametalized carbonbrush in current-carrying relation to the element, a non-oxidizing gassurrounding the device and means for introducing mercury vapor into thegas.

ROBERT M. BAKER. LLOYD SMEDE.

